In a heating, ventilating, and air conditioning (HVAC) system, air flow is typically controlled using- resistors to slow down the flow of air at different points to obtain the proper air balance throughout a building. These resistors may be comprised of gate valves, butterfly valves or dampers, and may be fixed, adjustable or motorized. When one resistor is adjusted, the pressure level throughout the HVAC system will change; any change in the HVAC system pressure will affect the flow of air past every other resistor. Thus, adjusting a resistor at the output causes "cross-talk." Previous attempts to solve the problem of air flow control have automated the resistors using microprocessors and servo-motors.
Municipal gas companies in the United States distribute gas through a network that is terminated with pressure regulators. In these gas distribution systems the pressure at the point of use is fairly independent of pressure changes throughout the distribution network. This can be accomplished because the distribution network is designed to withstand large pressures, and a large pressure drop can be caused at the point of use.
The approach, taken by gas companies, of providing a pressure regulator at the point of use has not been practical for the HVAC industry, because the HVAC industry moves very large quantities of air at very low pressure, and because the HVAC industry is usually more interested in controlling mass flow, not pressure. The comfort of the environment is determined by the thermal mass of hot and cold air that is moved.
Safety valves used in the gas industry, and in other fields involving the handling of fluids under high pressure, open or close only in extreme situations where a large rise or drop in pressure poses a danger. (Gas companies have safety valves that shut off the flow of gas when there is a large decrease in pressure, since such a decrease may be due to a leak downstream of the valve. Many safety valves vent fluid from a conduit when there is a large increase in pressure in order to prevent the pressure in the conduit from increasing beyond the bursting point of the conduit, or beyond the capability of machinery connected to the conduit.) Other valves such as those used in gasoline pumps, also shut off flow automatically when the backpressure increases to a certain point, indicating that the tank being filled is full. These safety valves and gasoline-pump valves are designed to be either fully opened or fully closed, and are not designed to precisely regulate the fluid flow.
One of the most complex problems confronted by the HVAC industry is controlling clean rooms. Clean room requirements dictate that the environment be kept at a constant temperature and humidity (typically within a few degrees and a few percent), that the mass flow into the environment be kept constant, and that the flow be distributed evenly across a ceiling. Clean room ceilings are constructed with special filters designed to remove very small particles from the air entering the room. In addition to being clean, the air leaving the filter should be at an exact velocity. The ceilings are designed to disperse the air into the clean room at the same velocity over the entire surface of the ceiling. The ceilings and filters are constructed to add as little resistance to the air flow as possible, and so that there is only little variation from one filter to the next.
In order to deliver the same mass flow to each filter, the HVAC industry uses a network of resistors deployed throughout the air delivery system. The air flow through each filter is controlled by adding or removing resistance. In a single clean room the ceiling may contain as many as 150 filters. A process called balancing is used to adjust the filter flow rates. The resistors are repeatedly adjusted in sequence, until the flow rate is within the specified range, or until the amount of time the clean room is down, during the balancing process, gets too expensive. After the balancing is completed, the whole network is still subject to changes in the supply pressure and the demand requirements of the clean room.